Powered model railway systems (such as model railroads) that use a digital command control (DCC) signal are prone to short-circuit and overload conditions that interrupt operation of the railway system or damage expensive power supplies and/or electrical components. The potential for track shorts increases as the number of operators on a system increases. Several solutions have been proposed to address the problem of track short circuits. Ballast lamps, relays, and kill switches have been used with limited success.
Some overloads conditions are caused by large capacitors used in sound system decoders. The overload condition appears as a short circuit until the capacitors in the decoders are charged. These types of overload conditions are short-lived and do not cause harm to other equipment.
The problem is that all good power supplies, whether conventional DC, DCC-based, or AC, have built in circuit protection that senses a short and shuts down all the power in about 0.25 second. The above-mentioned approaches, while protecting the power supply, shuts down all activity on the railway system until each and every operator resets their locomotives.
What is needed is a device that can determine if the overload condition is caused by a true short circuit in the system or just a temporary overload.